1. Field of the Invention
The present invention relates to batteries, and in particular, to the monitoring of battery capacity.
2. Background Art
A battery is a device that provides electrical energy used to power an electrical device. A battery typically includes one or more electrochemical cells that store chemical energy, which is converted to electrical energy output by the device to provide power. Batteries are used in a multitude of electrical devices, such as electrical devices that are mobile, are small, and/or are unable to be constantly connected to another power source such as AC (alternating current) power. Batteries may also be used in electrical devices as a backup power source, to provide power when a primary power source is lost.
A rechargeable battery is a type of battery that is becoming increasingly popular. A rechargeable battery can be restored to full charge by the application of electrical energy. Rechargeable batteries based on lithium, such as lithium ion and lithium polymer batteries, are becoming increasingly widespread. A typical charging cycle for a lithium rechargeable battery includes a first charge phase, where a constant current is used to charge the battery (while battery voltage increases), and a second charge phase, where a constant voltage is applied to the battery to finish charging the battery (while the charge current decreases). Typically, the maximum amount of charge that a lithium battery can maintain decreases with age.
Techniques exist for determining a stored charge in batteries (battery “state of charge”), and for determining an overall charge storage capacity of batteries (battery “state of health”). Conventional techniques for determining battery state of health measure an accumulated charge occurring during an entire charge cycle for the battery. Such techniques have disadvantages. For example, it is difficult to accurately measure the charge being stored in the battery. In particular, with regard to a lithium battery, the level of current entering the battery is very small towards the end of the constant voltage charging phase. For example, this amount of current may be less than 5 mA for a 100 mAH battery. Any inaccuracy in the current measuring components will cause a measurement of this small current to be incorrect. Thus, estimations of state of health performed while charging a battery can be inaccurate. In a similar fashion, estimations of battery state of health performed while discharging a battery are also known to be inaccurate.
Conventional circuits used to determine battery state of health are relatively complex, which may be undesirable, particularly in smaller sized devices. For example, a fuel gauging resistor and an analog to digital converter (ADC) are typically needed to track the charge current during the entire charging cycle. Such components undesirably add to the cost and complexity of the electrical device that uses the battery.
What is desired are ways of determining battery state of health that are more accurate and less complex than conventional techniques.